Varifocal system using hybrid tunable liquid crystal lenses

ABSTRACT

A device includes a first-type liquid crystal (LC) lens configured to provide a first optical power that is variable in a first step resolution. The device also includes a second-type LC lens coupled with the first-type LC lens, and configured to provide a second optical power that is variable in a second step resolution. The first step resolution is smaller than the second step resolution. A total optical power of the device is a sum of the first optical power and the second optical power, and is variable in the first step resolution.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/271,344, entitled “VARIFOCAL SYSTEM USING HYBRID TUNABLE LIQUIDCRYSTAL LENSES,” filed on Feb. 8, 2019, which claims the benefit of U.S.Provisional Application No. 62/712,001, filed Jul. 30, 2018. Contents ofthe above-mentioned applications are incorporated herein by reference intheir entirety.

BACKGROUND

The present disclosure generally relates to display technologies and,specifically, relates to a varifocal system based on hybrid tunablecrystal lenses.

Virtual reality (VR) headsets can be used to simulate virtualenvironments. For example, stereoscopic images can be displayed on anelectronic display inside a headset to simulate the illusion of depth,and head tracking sensors can be used to estimate what portion of thevirtual environment is being viewed by the user. However, becauseexisting headsets are often unable to correctly render or otherwisecompensate for vergence and accommodation conflicts, such simulation cancause visual fatigue and nausea of the users.

Augmented Reality (AR) headsets display a virtual image overlapping withreal world images. To create comfortable viewing experience, the virtualimage generated by the AR headsets needs to be displayed at the rightdistance for the eye accommodations of the real world images in realtime during the viewing process.

BRIEF SUMMARY OF THE DISCLOSURE

One aspect of the present disclosure provides a varifocal system. Thevarifocal system comprises a stacked first-type liquid crystal (LC) lensstructure and a stacked second-type LC lens structure in optical series.The stacked first-type LC lens structure includes a plurality offirst-type LC lenses, and a first-type LC lens of the plurality offirst-type LC lenses provides continuously variable optical states in afirst step resolution. The stacked second-type LC lens structureincludes a plurality of second-type LC lenses, and provides a pluralityof optical states in a second step resolution. The first step resolutionis smaller than the second step resolution, such that when the stackedsecond-type LC lens structure is switched between two optical states,the first-type LC lenses provide a continuous adjustment of opticalpower between the two optical states. The stacked first-type LC lensstructure and the stacked second-type LC lens structure together providea continuous adjustment range of optical power for the varifocal system.

Another aspect of the present disclosure provides a driving method for avarifocal system. The driving method comprises: stacking a plurality offirst-type liquid crystal (LC) lenses to form a stacked first-type LClens structure, wherein a first-type LC lens of the plurality offirst-type LC lenses provides continuously variable optical states in afirst step resolution; stacking a plurality of second-type LC lenses toform a stacked second-type LC lens structure arranged in optical serieswith the stacked first-type LC lens structure, wherein the stackedsecond-type LC lens structure provides a plurality of optical states ina second step resolution, and the second step resolution is larger thanthe first step resolution; determining a current optical state of thevarifocal system; determining a next optical state required by thevarifocal system in terms of the first step resolution and the secondstep resolution; switching the first-type LC lenses to provide acontinuous adjustment of optical power to achieve the next optical statein terms of the first step resolution; and switching the stackedsecond-type LC lens structure to achieve the next optical state in termsof the second step resolution, such that the stacked first-type LC lensstructure and the stacked second-type LC lens structure together providea continuous adjustment range from the current optical state to the nextoptical state for the varifocal system.

Other aspects of the present disclosure can be understood by thoseskilled in the art in light of the description, the claims, and thedrawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are provided for illustrative purposes accordingto various disclosed embodiments and are not intended to limit the scopeof the present disclosure.

FIG. 1A shows the relationship between vergence and eye focal length inthe real word of the present disclosure;

FIG. 1B shows the conflict between vergence and eye focal length in athree dimensional (3D) display screen of the present disclosure;

FIG. 2A is a wire diagram of a head-mounted display, in accordance withan embodiment of the present disclosure;

FIG. 2B is a cross section of a front rigid body of the head-mounteddisplay in FIG. 2A, in accordance with an embodiment of the presentdisclosure;

FIG. 3A is an example of Pancharatnam Berry Phase (PBP) liquid crystal(LC) lens, in accordance with an embodiment of the present disclosure;

FIG. 3B is an example of liquid crystal orientations in the PBP LC lensin FIG. 3A, in accordance with an embodiment of the present disclosure;

FIG. 3C is a portion of liquid crystal orientations in the PBP LC lensin FIG. 3A, in accordance with an embodiment of the present disclosure;

FIG. 4A is a diagram of a varifocal structure using hybrid tunable LClenses, in accordance with an embodiment of the present disclosure;

FIG. 4B is an example of a first LC lens in FIG. 4A, in accordance withan embodiment of the present disclosure;

FIG. 4C is an example of the switched-off first LC lens and theswitched-off second LC lens in FIG. 4A, in accordance with an embodimentof the present disclosure;

FIG. 4D is an example of the switched-on first LC lens and theswitched-off second LC lens in FIG. 4A, in accordance with an embodimentof the present disclosure;

FIG. 4E is an example of the switched-off first LC lens and theswitched-on second LC lens in FIG. 4A, in accordance with an embodimentof the present disclosure;

FIG. 4F is an example of a stacked PBP LC lens structure in FIG. 4A, inaccordance with an embodiment of the present disclosure;

FIG. 5A is an example of a driving scheme of the varifocal structure inFIG. 4A, in accordance with an embodiment of the present disclosure;

FIG. 5B is an example of an optical path of the varifocal structurehaving the driving scheme in FIG. 5A, in accordance with an embodimentof the present disclosure;

FIG. 5C is a table showing example optical adjustments in a positiverange of the varifocal structure having the driving scheme in FIG. 5A,in accordance with an embodiment of the present disclosure;

FIG. 6A is another example of a driving scheme of the varifocalstructure in FIG. 4A, in accordance with an embodiment of the presentdisclosure;

FIG. 6B is an example of an optical path of the varifocal structurehaving the driving scheme in FIG. 6A, in accordance with an embodimentof the present disclosure;

FIG. 6C is a table showing example optical adjustments in a negativerange of the varifocal structure having the driving scheme in FIG. 6A,in accordance with an embodiment of the present disclosure;

FIG. 7 is varifocal system in which a HMD operates, in accordance withan embodiment of the present disclosure;

FIG. 8 is a process for mitigating vergence-accommodation conflict byadjusting the focal length of a HMD, in accordance with an embodiment ofthe present disclosure; and

FIG. 9 shows an example process for mitigating vergence-accommodationconflict by adjusting a focal length of a varifocal block that includesvarifocal structures, in accordance with an embodiment of the presentdisclosure.

DETAILED DESCRIPTION

A varifocal system includes a head-mounted display (HMD). The HMDincludes a varifocal block. The HMD presents content via an electronicdisplay to a wearing user at a focal distance. The varifocal blockadjusts the focal distance in accordance with instructions from the HMDto, e.g., mitigate vergence accommodation conflict of eyes of thewearing user. The focal distance is adjusted by adjusting an opticalpower associated with the varifocal block, and specifically by adjustingthe optical powers associated with one or more varifocal structureswithin the varifocal block.

A varifocal structure is an optical device that is configured todynamically adjust its focus in accordance with instructions from thevarifocal system. The varifocal block include ones or more varifocalstructures in optical series. In the disclosed embodiments, thevarifocal structure includes a stacked first-type liquid crystal (LC)lens structure and a stacked second-type LC lens structure in opticalseries. The stacked first-type LC lens structure includes a plurality offirst-type LC lenses, each of which utilizes the change in polar angle(or tilt angle) of LC molecules to create a lens profile and providecontinuously variable focal states with a first step resolution. Thestacked second-type LC lens structure includes a plurality ofsecond-type LC lenses utilizing the change in azimuthal angle of LCmolecules to create a lens profile. The stacked second-type LC lensstructure provides a plurality of discrete focal states with a secondstep resolution. The first step resolution is smaller than the secondstep resolution, such that when the stacked second-type LC lensstructure is switched between two optical states, the first-type LClenses provide a continuous adjustment of optical power between the twooptical states, and the stacked first-type LC lens structure and thestacked second-type LC lens structure together provide a continuousadjustment range of optical power for the varifocal structure.

Optical series refers to relative positioning of a plurality of opticalelements, such that light, for each optical element of the plurality ofoptical elements, is transmitted by that optical element before beingtransmitted by another optical element of the plurality of opticalelements. Moreover, ordering of the optical elements does not matter.For example, optical element A placed before optical element B, oroptical element B placed before optical element A, are both in opticalseries. Similar to electric circuitry design, optical series representsoptical elements with their optical properties compounded when placed inseries.

A PBP LC lens may be active or passive. An active PBP LC lens is anoptical element that has three discrete focal states (also referred toas optical states). The three optical states are an additive state, aneutral state, and a subtractive state. The additive state adds opticalpower to the system (i.e., has a positive focus of ƒ), the neutral statedoes not affect the optical power of the system (and does not affect thepolarization of light passing through the PBP LC lens), and thesubtractive state subtracts optical power from the system (i.e., has anegative focus of ƒ).

The state of an active PBP LC lens is determined by the by thehandedness of polarization of light incident on the active PBP LC lensand an applied voltage. An active PBP LC lens operates in a subtractivestate responsive to incident light with a left-handed circularpolarization and an applied voltage of zero (or more generally belowsome minimal value), operates in an additive state responsive toincident light with a right-handed circular polarization and the appliedvoltage of zero (or more generally below some minimal value), andoperates in a neutral state (regardless of polarization) responsive toan applied voltage larger than a threshold voltage which aligns liquidcrystal with positive dielectric anisotropy along with the electricfield. Note that when the active PBP LC lens is in the additive orsubtractive state, light output from the active PBP LC lens has ahandedness opposite to that of the light input into the active PBP LClens. In contrast, when the active PBP LC lens is in the neutral state,light output from the active PBP LC lens has the same handedness as thelight input into the active PBP LC lens.

In contrast, a passive PBP LC lens has two optical states, specifically,an additive state and a subtractive state. The state of a passive PBP LClens is determined by the handedness of polarization of light incidenton the passive PBP LC lens. A passive PBP LC lens operates in asubtractive state responsive to incident light with a left-handedpolarization, and operates in an additive state responsive to incidentlight with a right-handed polarization. Note that the passive PBP LClens outputs light that has a handedness opposite to that of the lightinput into the passive PBP LC lens.

The stacked first-type LC lens structure may include a first LC lensproviding a variable optical power to linearly polarized light having afirst polarization direction, and a second LC lens providing a variableoptical power to linearly polarized light having a second polarizationdirection perpendicular to the first direction. The first LC lens andthe second LC lens are optical elements which utilize the change inpolar angle (or tilt angle) of LC molecules to create a lens profile,i.e., a refractive index profile.

To generate a desired refractive index profile, various electrodestructures and addressing approaches have been introduced to the LClens, such as a set of the discrete ring-patterned electrodes addressedindividually with different voltages, the spatial distribution ofelectric field on a hole-patterned electrode plate to control the indexprofile, or a spherical shape of the electrode, which can be addressedto tune the optical power continuously. That is, the LC lens whichutilizes the change in polar angle (or tilt angle) to create the lensprofile can provide continuously variable focal states. In contrast, thePBP LC lens, which utilizes the change in azimuthal angle to create alens profile, only provides a plurality of discrete focal states, forexample, an active PBP LC lens provides 3 focal states, a passive PBP LClens provides 2 focal states.

The polarization converter is an active polarization converter whichconverts the polarization direction of the incident linearly polarizedlight from a first polarization direction to a second polaritonpolarization direction perpendicular to the first polarization directionor maintains the polarization direction of linearly polarized light inaccordance with a switching state (i.e., active or non-active). Incertain embodiments, the polarization converter may be a switchable halfwaveplate (SHWP). The linear-to-circular polarization converter covertslinearly polarized light with orthogonal polarization directions tocircularly polarized light having the same handedness in accordance witha switching state (i.e., active or non-active). In certain embodiments,the linear-to-circular polarization converter may include an activepolarization converter and a quarter waveplate (QWP).

In certain embodiments, a virtual object is presented on the electronicdisplay of the HMD that is part of the varifocal system. The lightemitted by the HMD is configured to have a particular focal distance,such that the virtual scene appears to a user at a particular focalplane. As the content to be rendered moves closer/farther from the user,the HMD correspondingly instructs the varifocal block to adjust thefocal distance to mitigate a possibility of a user experiencing aconflict with eye vergence and eye accommodation. Additionally, incertain embodiments, the HMD may track a user's eyes such that thevarifocal system is able to approximate gaze lines and determine a gazepoint including a vergence depth (an estimated point of intersection ofthe gaze lines) to determine an appropriate amount of accommodation toprovide the user. The gaze point identifies an object or plane of focusfor a particular frame of the virtual scene and the HMD adjusts thedistance of the varifocal block to keep the user's eye in a zone ofcomfort as vergence and accommodation change.

Vergence-accommodation conflict is a problem in many virtual realitysystems. Vergence is the simultaneous movement or rotation of both eyesin opposite directions to obtain or maintain single binocular vision andis connected to accommodation of the eye. Under normal conditions, whenhuman eyes look at a new object at a distance different from an objectthey had been looking at, the eyes automatically change focus (bychanging their shape) to provide accommodation at the new distance orvergence depth of the new object.

FIG. 1A shows an example of how the human eye experiences vergence andaccommodation in the real world. As shown in FIG. 1A, the user islooking at a real object 100 (i.e., the user's eyes are verged on thereal object 100 and gaze lines from the user's eyes intersect at realobject 100.). As the real object 100 is moved closer to the user, asindicated by the arrow in FIG. 1A, each eye 102 rotates inward (i.e.,convergence) to stay verged on the real object 100. As the real object100 gets closer, the eye 102 must “accommodate” for the closer distanceby changing its shape to reduce the power or focal length. Thus, undernormal conditions in the real world, the vergence depth (d_(v)) is equalto the focal length (d_(ƒ)).

FIG. 1B shows an example conflict between vergence and accommodationthat can occur with some three-dimensional displays. As shown in FIG.1B, a user is looking at a virtual object 100B displayed on anelectronic screen 104. However, the user's eyes are verged on and gazelines from the user's eyes intersect at virtual object 100B, which is agreater distance from the user's eyes than the electronic screen 104. Asthe virtual object 100B is rendered on the electronic display 104 toappear closer to the user, each eye 102 again rotates inward to stayverged on the virtual object 100B, but the power or focal length of eacheye is not reduced; hence, the user's eyes do not accommodate as in FIG.1A. Thus, instead of reducing power or focal length to accommodate forthe closer vergence depth, each eye 102 maintains accommodation at adistance associated with the electronic display 104. Thus, the vergencedepth (dv) often is not equal to the focal length (df) for the human eyefor objects displayed on 3D electronic displays. This discrepancybetween vergence depth and focal length is referred to as“vergence-accommodation conflict.” A user experiencing only vergence oraccommodation and not both will eventually experience some degree offatigue and nausea, which is undesirable for virtual reality systemcreators.

FIG. 2A is a wire diagram of a HMD 200, in accordance with anembodiment. As shown in FIG. 2A, the HMD 200 may include a front rigidbody 205 and a band 210. The front rigid body 205 may include one ormore electronic display elements of an electronic display (not shown),an inertial measurement unit (IMU) 215, one or more position sensors220, and locators 225. In the embodiment shown by FIG. 2A, the positionsensors 220 may be located within the IMU 215, and neither the IMU 215nor the position sensors 220 may be visible to the user. The IMU 215,the position sensors 220, and the locators 225 may be discussed indetail below with regard to FIG. 7. Note in embodiments, where the HMD200 acts as an AR or MR device portions of the HMD 200 and its internalcomponents may be at least partially transparent.

FIG. 2B is a cross section 250 of the front rigid body 205 of theembodiment of the HMD 200 shown in FIG. 2A. As shown in FIG. 2B, thefront rigid body 205 may include an electronic display 255 and avarifocal block 260 that together provide image light to an exit pupil263. The exit pupil 263 may be the location of the front rigid body 205where a user's eye 265 is positioned. For purposes of illustration, FIG.2B shows a cross section 250 associated with a single eye 265, butanother varifocal block 260, separate from the varifo1cal block 260,provides altered image light to another eye of the user. Additionally,the HMD 200 may include an eye tracking system (not shown). The eyetracking system may include, e.g., one or more sources that illuminateone or both eyes of the user, and one or more cameras that capturesimages of one or both eyes of the user.

The electronic display 255 may display images to the user. In variousembodiments, the electronic display 255 may comprise a single electronicdisplay or multiple electronic displays (e.g., a display for each eye ofa user). Examples of the electronic display 255 include: a liquidcrystal display (LCD), an organic light-emitting diode (OLED) display,an active-matrix organic light-emitting diode display (AMOLED), aquantum dot organic light-emitting diode (QOLED), a quantum dotlight-emitting diode (QLED), some other display, or some combinationthereof.

The varifocal block 260 may adjust an orientation from light emittedfrom the electronic display 255, such that it appears at particularfocal distances from the user. The varifocal block 260 may include oneor more varifocal structures in optical series. A varifocal structure isan optical device that is configured to dynamically adjust its focus inaccordance with instructions from a varifocal system. In the disclosedembodiments, the varifocal structure may include a polarizationconverter converting incident linearly polarized light between a firstpolarization direction and a second polarization direction, a firstliquid crystal lens in response to linearly polarized light having thefirst polarization direction, a second liquid crystal lens in responseto linearly polarized light having the second polarization direction, alinear-to-circular polarization converter, and a stacked PancharatnamBerry Phase (PBP) liquid crystal lens structure in optical series. Thevarifocal structure may also include one or more substrate layers, alinear polarizer, or some combination thereof. For example, the linearpolarizer may be optically coupled to the polarization converter, toensure the light incident onto the polarization converter is incidentlinearly polarized light having the first polarization direction or thesecond polarization direction.

The substrate layers are layers which other elements (e.g., SHWP, liquidcrystal, etc.) may be formed upon, coupled to, etc. The substrate layersare substantially transparent in the visible band (−380 nm to 750 nm).In certain embodiments, the substrate may also be transparent in some orall of the infrared (IR) band (−750 nm to 1 mm). The substrate layersmay be composed of, e.g., SiO₂, plastic, sapphire, etc.

Additionally, in certain embodiments, the varifocal block 260 maymagnify received light, corrects optical errors associated with theimage light, and presents the corrected image light is presented to auser of the HMD 200. The varifocal block 260 may additionally includeone or more optical elements in optical series. An optical element maybe an aperture, a Fresnel lens, a convex lens, a concave lens, a filter,or any other suitable optical element that affects the blurred imagelight. Moreover, the varifocal block 260 may include combinations ofdifferent optical elements. In certain embodiments, one or more of theoptical elements in the varifocal block 260 may have one or morecoatings, such as anti-reflective coatings.

FIG. 3A is an example of PBP LC lens 300, in accordance with anembodiment. As shown in FIG. 3A, the PBP LC lens 300 may create arespective lens profile via an in-plane orientation (θ, azimuth angle)of a liquid crystal molecule, in which the phase difference T=2θ. Incontrast, a conventional liquid crystal lens creates a lens profile viaa birefringence (An) and layer thickness (d) of liquid crystals, and anumber(#) of Fresnel zones (if it is Fresnel lens design), in which thephase difference T=dΔn *#*2π/λ. Accordingly, in certain embodiments, thePBP LC lens 300 may have a large aperture size and may be made with avery thin liquid crystal layer, which allows fast switching speed toturn the lens power on/off.

Design specifications for HMDs used for VR, AR, or MR applicationstypically requires a large range of optical power to adapt for human eyevergence-accommodation (e.g. ˜±2 Diopters or more), fast switchingspeeds (e.g., ˜300 ms), and a good quality image. Note conventionalliquid crystal lenses may be not well suited to these applications,because a conventional liquid crystal lens generally would require theliquid crystal materials to have a relatively high index of refractionor be relatively thick (which reduces switching speeds). In contrast, aPBP LC lens is able to meet design specs using liquid crystal materialshaving a relatively low index of refraction and, moreover, the PBP LClens is thin (e.g., a single liquid crystal layer can be ˜2 μm) and hashigh switching speeds (e.g., 300 ms).

FIG. 3B is an example of liquid crystal orientations 310 in the PBP LClens 300 of FIG. 3A, in accordance with an embodiment. As shown in FIG.3B, in the PBP LC lens 300, an azimuth angle (θ) of a liquid crystalmolecule may be continuously changed from a center 320 of the liquidcrystal lens 300 to an edge 330 of the PBP LC lens 300, with a variedpitch Λ. Pitch is defined in a way that the azimuth angle of LC isrotated 180° from the initial state.

FIG. 3C is a section of liquid crystal orientations 340 taken alongy-axis in the PBP LC lens 300 of FIG. 3A, in accordance with anembodiment. As shown in FIG. 3C, it is apparent from the liquid crystalorientation 340 that a rate of pitch variation may be a function ofdistance from the lens center 32θ. The rate of pitch variation mayincrease with distance from the lens center. For example, the pitch atthe lens center 320 (Λ₀) is the slowest, and the pitch at the edge 330(Λr) is the highest, i.e., Λ₀>Λ₁>. . . >Λ_(r). In the x-y plane, to makea PBP LC lens with lens radius (r) and lens power (+/−ƒ), the azimuthangle θ may meet: 2θ=r²/f*(π/λ), where λ, is the wavelength of light.Along with the z-axis, a dual twist or multiple twisted structure layersoffers achromatic performance on efficiency in the PBP LC lens 300.Along with the z-axis, the non-twisted structure is simpler to fabricatethen a twisted structure, but is optimized for a monochromatic light.

Note that a PBP LC lens may have a twisted or non-twisted structure. Incertain embodiments, a stacked PBP LC lens structure may include one ormore PBP LC lenses having a twisted structure, one or more PBP LC lenseshaving a non-twisted structure, or some combination thereof.

Although a PBP LC lens is able to meet design specs of HMDs, a PBP LClens and a PBP LC lens stack may only provide a plurality of discretefocal states. As a result, when an individual PBP LC lens has asubstantially large optical power or a PBP LC lens stack has asubstantially large step resolution (e.g., 0.5 Diopters), obvious imagedistortion may be perceived by human eyes during the switching betweenthe discrete focal states, which degrades the viewing experience. It ispossible to fabricate a PBP LC lens having a substantially small opticalpower (e.g., 0.05 Diopters), then the image distortion happened duringthe switching between the discrete focal states may be too small to beperceived by human eyes. However, a large number of PBP LC lenses mayhave to be stacked together to obtain a desired range of optical powerto adapt for human eye vergence-accommodation (e.g., ˜±2 Diopters ormore) in HMDs. Accordingly, the PBP LC lens stack may be substantiallythick.

As discussed above, an LC lens which utilizes the change in polar angle(or tilt angle) to create the lens profile can provide visuallycontinuously variable focal states, because the step resolution of theLC lens is too small to be perceived by human eyes, for example, thestep resolution of the LC lens may be smaller than 1/10 of the stepresolution of a stacked PBP LC lens structure. In view of this, thepresent disclosure provides a varifocal structure based on hybridtunable liquid crystal lenses, which include first and second liquidcrystal lenses utilizing the change in polar angle (or tilt angle) of LCmolecules to create a lens profile, and a stacked PBP LC lens structurein optical series.

The stacked PBP LC lens structure may provide a plurality of discretefocal states, and the first and second liquid crystal lenses mayalternately provide continuously variable focal states when switchingamong the discrete focal states of the stacked PBP LC lens structure.The stacked PBP LC lens structure, the first LC lens, and the second LClens together may provide continuously variable focal states (i.e., acontinuous range of adjustment of optical power) for the varifocalstructure. Thus, when switching among the discrete focusing states ofthe stacked PBP LC lens structure, the image distortion caused by largestep resolution of the PBP LC lens stack may be suppressed, and smoothertransition between different focal states may be perceived by the humaneyes.

Below various designs of varifocal structures are discussed. It isimportant to note that these designs are merely illustrative, and otherdesigns of varifocal structures may be generated using the principlesdescribed herein. In certain embodiments, the varifocal structureswithin the varifocal block 260 may be designed to meet requirements foran HMD (e.g., the HMD 200). Design requirements may include, forexample, large aperture size (e.g., 2:4 cm) for large field of view(e.g., FOV, −90 degrees with 20 mm eye relief distance), large opticalpower (e.g., ±2.0 Diopters) for adapting human eye vergenceaccommodation, and fast switching speed (−300 ms) for adapting human eyevergence-accommodation, and good image quality for meeting human eyeacuity. In certain other embodiments, the varifocal structures mayinclude other optical elements in optical series.

FIG. 4A is a diagram of a varifocal structure including hybrid tunableliquid crystal lenses, in accordance with an embodiment. As shown inFIG. 4A, the varifocal structure may include a linear polarizer 410, apolarization converter 42θ, a first liquid crystal (LC) lens 430, asecond LC lens 440, a linear-to-circular polarization converter 450, anda stacked Pancharatnam Berry Phase (PBP) LC lens structure 480 inoptical series.

In particular, the linear polarizer 410 may transmit linearly polarizedlight with a particular polarization direction, for example, a firstpolarization direction. In certain embodiments, the linear polarizer 410may be omitted when the light-in is linearly polarized light having thefirst polarization direction. The polarization converter 420 may be anactive polarization converter which converts or maintains thepolarization direction of linearly polarized light in accordance with aswitching state (i.e., active or non-active). The switching state of thepolarization converter 420 is either active or non-active. For example,when active, the polarization converter 420 may convert the polarizationdirection of linearly polarized light from the first polarizationdirection to a second polarization direction perpendicular to the firstpolarization direction. When non-active, the polarization converter 420may directly transmit the linearly polarized light having the firstpolarization direction without affecting the polarization direction. Incertain embodiments, the polarization converter 420 may be a switchablehalf waveplate (SHWP).

The first LC lens 430 and the second LC lens 440 each may be an LC lenswhich utilizes the change in polar angle (or tilt angle) to create thelens profile, and provide continuously variable focal states. In certainembodiments, the first LC lens 430 and the second LC lens 440 may havethe same structure but arranged in a specific manner, such that one ofthe first LC lens 430 and the second LC lens 440 may be configured toprovide an adjustable range of optical power (i.e., continuouslyvariable focal states) for linearly polarized light having the firstpolarization direction, and the other may be configured to provide anadjustable range of optical power (i.e., continuously variable focalstates) for linearly polarized light having the second polarizationdirection.

FIG. 4B is an example of a first LC lens 430 in FIG. 4A, in accordancewith an embodiment. As shown in FIG. 4B, the first LC lens 430 mayinclude an LC cell formed by two substrates 402 (e.g., glasssubstrates). Transparent indium tin oxide (ITO) electrodes 404 may bedisposed on opposing surfaces of the substrates 402 to apply an electricfield. The ITO electrodes 404 may include a planar electrode and aring-shaped electrode, respectively. A polyimide alignment layer 406 maybe coated on each substrate 402 and rubbed along one direction to enablea preferred orientation of LC molecules 408 in the LC cell.

After a voltage is applied to the LC cell, due to the ring-shaped ITOelectrode disposed on the substrate 402, from the center to the edge ofthe LC cell, the electrical field may gradually increase and, thus, theorientation of LC directors 414 may change from being parallel to thesurface of the substrate 402 to being closer to perpendicular to thesurface of the substrate 402. Accordingly, for the incident light havingan x-direction polarization direction, the effective refractive index ofthe LC molecules 408 may gradually change from the center to the edge ofthe LC cell. Thus, a positive lens profile may be obtained, i.e., theformed LC lens may be a positive LC lens having positive optical power.Because the lens profile can be continuously adjusted along with thecontinuously varied voltage applied to the LC cell, the formed LC lensmay be able to provide a continuous range of adjustment of the opticalpower, i.e., continuously variable focal states.

It should be noted that, FIG. 4B shows an LC lens structure based onlight refraction, which is merely for illustrative purposes and is notintended to limit the scope of the present disclosure. The first LC lens430 and the second LC lens 440 may have any appropriate structure whichutilize the change in polar angle (or tilt angle) to create the lensprofile and provide continuously variable focal states, such as aFresnel LC lens based on light diffraction. In certain embodiments, thefirst LC lens 430 and/or the second LC lens 440 may be a negative lensproviding continuous variable negative optical power. In certainembodiments, the first LC lens 430 and/or the second LC lens 440 mayprovide continuous variable negative optical power and positive opticalpower. In certain embodiments, the first LC lens 430 and the second LClens 440 may also have different lens structures.

To enable one of the first LC lens 430 and the second LC lens 440 toprovide continuously variable focal states for linearly polarized lighthaving the first polarization direction and the other to providecontinuously variable focal states for linearly polarized light havingthe second polarization direction perpendicular to the firstpolarization direction, in certain embodiments, the alignment direction(i.e., the rubbing direction) of the first LC lens 430 may beperpendicular to the alignment direction of the second LC lens 440. Acorresponding structure is shown in FIGS. 4C-4E.

FIG. 4C is an example of the switched-off first LC lens and theswitched-off second LC lens in FIG. 4A, in accordance with anembodiment, FIG. 4D is an example of the switched-on first LC lens andthe switched-off second LC lens in FIG. 4A, in accordance with anembodiment, and FIG. 4E is an example of the switched-off first LC lensand the switched-on second LC lens in FIG. 4A, in accordance with anembodiment.

As shown in FIGS. 4C-4E, both the first LC lens 430 and second LC lens440 may have a homogeneous alignment, the alignment directions (i.e.,the rubbing directions) 412 of the first LC lens 430 may be along ±xdirection, the alignment direction 412′ of the second LC lens 440 may bealong ±y direction. The light propagation direction may be along+zdirection.

Thus, in one embodiment, provided the first polarization direction isalong the x−direction and the second polarization direction is along they-direction, i.e., the linear polarized light having the firstpolarization direction is p-polarized light and the linear polarizedlight having the second polarization direction is s-polarized light, thefirst LC lens 430 may provide the continuously variable focal states forthe linearly polarized light having the first polarization direction (x−direction) as the applied voltage continuously varies, while appear tobe a transparent plate to the linearly polarized light having the secondpolarization direction (y− direction), as FIG. 4D shows. Meanwhile, thesecond LC lens 440 may provide continuously variable focal states forlinearly polarized light having the second polarization direction (y−direction) as the applied voltage continuously varies, while appear tobe a transparent plate to the linearly polarized light having the firstpolarization direction(x− direction), as FIG. 4E shows.

It should be noted that, FIGS. 4C-4E shows the first the first LC lens430 and the second LC lens 440 are separated from each other by acertain distance, which is for illustrative purposes and is not intendedto limit the scope of the present disclosure. The first LC lens 430 andthe second LC lens 440 may be arrange without any gap, and/or may sharecertain substrates with each other.

Returning to FIG. 4A, the linear-to-circular polarization converter 450may covert linearly polarized light to circularly polarized light. Incertain embodiments, the linear-to-circular polarization converter mayinclude an polarization converter 460 and a quarter waveplate (QWP) 470.Similar to the polarization converter 420, the polarization converter460 may be an active polarization converter, e.g., a switchable halfwaveplate (SHWP), which converts or maintains the polarization directionof linearly polarized light in accordance with a switching state (i.e.,active or non-active). The quarter waveplate 470 may convert thelinearly polarized light to circularly polarized light.

In one embodiment, when active, the polarization converter 460 mayconvert the linearly polarized light having the first polarizationdirection to the second polarization direction. When non-active, thepolarization converter 460 may directly transmit the linearly polarizedlight having the first polarization direction without affecting thepolarization direction. That is, regardless being active or non-active,the polarization converter 460 may always output the linearly polarizedlight having the first polarization direction. After passing the quarterwaveplate 470, the linearly polarized light having the firstpolarization direction may be converted to circularly polarized light.

In another embodiment, when active, the polarization converter 460 mayconvert the linearly polarized light having the second polarizationdirection to the first polarization direction. When non-active, thepolarization converter 460 may directly transmit the linearly polarizedlight having the second polarization direction without affecting thepolarization direction. That is, regardless being active or non-active,the polarization converter 460 may always output the linearly polarizedlight having the second polarization direction. After passing thequarter waveplate 470, the linearly polarized light having the secondpolarization direction may be converted to circularly polarized light.

That is, no matter whether the linear-to-circular polarization converter450 receives the linearly polarized light having the first polarizationdirection or the second polarization direction, the linear-to-circularpolarization converter 450 may always output circularly polarized lightwith a same handedness. The stacked PBP LC lens structure may include aplurality of PBP LC lens and at least one switchable half waveplate(SHWP) arranged adjacent to a PBP LC lens.

FIG. 4F is an example of a stacked PBP LC lens structure 480 in FIG. 4A,in accordance with an embodiment. The stacked PBP LC lens structure 480may be composed of alternating SHWPs and active elements. As shown inFIG. 4F, the stacked PBP LC lens structure 480 may include activeelements 421, 423, 425, 427, 429 and SHWPs 411, 413, 415, 417alternately arranged.

An active element is an active PBP LC lens. In an additive state, theactive element may add N*R of optical power, and in a subtractive state,the active element may subtract −N*R of optical power, where R (stepresolution) is any positive number (e.g., 0.1, 0.25, 0.5 etc., the unitof R is diopter) and N is a positive integer. The active elements 421,423, 425, 427, 429 included in the stacked PBP LC lens structure 480 mayprovide same or different optical power. In certain embodiments, theactive elements 421, 423, 425, 427, 429 each may provide optical powerof R in the additive state and −R in the subtractive state and, thus,the stacked PBP LC lens structure 480 may provide a range of opticalpower adjustment of −5R to 5R, in increments of R.

The SHWP 411, 413, 415, 417 may be a half waveplate that transmits aparticular handedness of polarized light in accordance with a switchingstate (i.e., active or non-active). A varifocal block may use the SHWPto control the handedness of polarization of light in accordance with aswitching state. The switching state of the SHWP is either active ornon-active. When active, the SHWP may reverse the handedness ofpolarized light, and when non-active, the SHWP may transmit polarizedlight without affecting the handedness. As discussed above, a PBP LClens acts in an additive state when receiving right-handed circularlypolarized (RCP) light, and conversely, acts in a subtractive state ifwhen receiving left-handed circularly polarized (LCP) light.Accordingly, a SHWP placed before a PBP LC lens in optical series may beable to control whether the PBP LC lens acts in an additive orsubtractive state by controlling the handedness of polarization of thelight incident onto the PBP LC lens.

As shown in FIG. 4F, the light-in 490 may be left-handed circularlypolarized (LCP) light or right-handed circularly polarized (RCP) light.The state of the SHWP 411, 413, 415, 417 may determine the handedness ofthe light output from the SHWP 411, 413, 415, 417. When not in a neutralstate, an active element reverses the handedness of circularly polarizedlight in addition to focusing/defocusing the incident light. Hence, whenthe light-in 490 is left-handed circularly polarized (LCP) light, theactive element 421 may output right-handed circularly polarized (RCP)light with a reduction of optical power of −R. When the light incidentonto the SHWP 411 is right-handed circularly polarized (RCP) light andthe SHWP 411 is active, the SHWP 411 may reverse the polarization toleft handedness, and when the light-in 490 is right-handed circularlypolarized (RCP) light and the SHWP 411 is non-active, the SHWP 411 maymaintain the polarization as right handed.

It should be noted that, the design of the stacked PBP LC lens structure480 are merely for illustrative purposes, and other designs of stackedPBP LC lens structures may be generated using the principles describedherein.

Returning to FIG. 4A, the first LC lens 430 and second LC lens 440 eachmay be configured to have a continuous adjustment range of optical powerequal to or larger than the step resolution (i.e., R) of the stacked PBPLC lens structure 480. Herein the continuous adjustment range of opticalpower of the LC lens refers to a range from the minimum optical power tothe maximum optical power of the LC lens.

During the operation of the varifocal structure, the stacked PBP LC lensstructure 480 may provide a plurality of discrete focal states, and thefirst LC lens 430 and second LC lens 440 may alternately providecontinuously variable focal states between two adjacent focal states ofthe stacked PBP LC lens structure 480. The stacked PBP LC lens structure480, the first LC lens 430, and the second LC lens 440 together mayprovide continuously variable focal states (i.e., a continuousadjustment range of optical power) for the varifocal structure 400.Thus, when switching among the plurality of discrete focusing states ofthe stacked PBP LC lens structure 480, the image distortion caused bylarge step resolution (i.e., R) of the stacked PBP LC lens structure 480may be suppressed, and smoother transition may be perceived by the humaneyes. The continuous adjustment range of optical power of the varifocalstructure 400 may be determined by the optical power of the stacked PBPLC lens structure 480, for example, a range from the maximum opticalpower to the minimum optical power of the stacked PBP LC lens structure.Details of the operation of the varifocal structure are discussed belowwith regard to FIGS. 5A-6C.

FIG. 5A is an example of a driving scheme of the varifocal structure inFIG. 4A, in accordance with an embodiment, FIG. 5B is an example of anoptical path of the varifocal structure having the driving scheme inFIG. 5A, in accordance with an embodiment, and FIG. 5C is a tableshowing example optical adjustments in a positive range of the varifocalstructure having the driving scheme in FIG. 5A, in accordance with anembodiment. The horizontal axis and the vertical axis in FIG. 5Arepresent time and applied voltage to an element in the varifocalstructure, respectively.

Referring to FIG. 5B, in certain embodiments, the light having the firstpolarization direction is p-polarized light and the light having thesecond polarization direction is s-polarized light. The polarizationconverter 420 may convert p-polarized light to s-polarized light inactive, and maintain p-polarized light in non-active. The polarizationconverter 460 may convert s-polarized light to p-polarized light inactive, and in non-active to maintain p-polarized light in non-active.In certain embodiments, the varifocal structure 400 may have an initialoptical power A, where A may be any appropriate number with a unit ofDiopter. For example, in the stacked PBP LC lens structure 480 of thevarifocal structure 400 in FIG. 5B, the active element 421 is configuredto have optical power of A in the additive state and—A in thesubtractive state, and the initial optical power of A of the varifocalstructure 400 is resulted from the active element 421. The activeelements 423, 425, 427, 429 each provides optical power of R in theadditive state and −R in the subtractive state, and the step resolutionof the stacked PBP LC lens structure 480 is R.

When the active elements 421, 423, 425, 427, 429 operate in a neutralstate (regardless of polarization) responsive to an applied voltagelarger than a threshold voltage which aligns LC molecules with positivedielectric anisotropy along with the electric field, the optical power,the active elements 421, 423, 425, 427, 429 provide zero optical power.The first LC lens 430 and the second LC lens 440 each may have acontinuous adjustment range of optical power equal to or larger than R.

Referring to FIGS. 5A-5C, during T1 stage (t0<t<t1), the polarizationconverter 420 may be non-active and output p-polarized light. The firstLC lens 430 may be switched-on to provide continuous variable opticalpower from 0 to R as the applied voltage gradually increases, and thesecond LC lens 440 may be switched off. The polarization converter 460included in the linear-to-circularly polarization converter 450 may benon-active and output p-polarized light, and the p-polarized light maybe converted to right-handed circularly polarized (RCP) light afterpassing the quarter waveplate 470 included in the linear-to-circularlypolarization converter 450. The active element (AE) 421 may receive theright-handed circularly polarized (RCP) light, thereby having theadditive state to provide optical power of A.

The active elements (AE) 423, 425, 427 and 429 may be in the neutralstate (i.e., zero optical power) and the SHWPs 411, 413, 415 and 417 maybe non-active without changing the handedness of the light incidentthereon. Thus, from 0 to t1, the total optical power of the varifocalstructure 400 may be continuously adjusted from A to A+R as the voltageapplied to the first LC lens 430 gradually increases.

At the time point t1, the first LC lens 430 may be switched off. TheSHWP 411 may become active, thereby changing the handedness of thecircularly light from left handedness to right handedness, and theactive element (AE) 423 may enter the additive state to provide opticalpower of R. The active element (AE) 421 may be in the additive state toprovide optical power of A. Thus, at the time point t1, the varifocalstructure 400 may have stable optical power of A+R.

During T2 stage (t1<t<t2), the polarization converter 420 may be activeand output s-polarized light, the second LC lens 440 may be switched-onto provide continuous variable optical power from 0 to R as the appliedvoltage gradually increases, and the first LC lens 430 may be switchedoff. That is, the first LC lens 430 and the second LC lens 440 may beswitched on and off alternatingly, and the first LC lens 430 and thesecond LC lens 440 may be considered as two independent channels toincrease/decrease the optical power. Certainly, a different number ofindependent channels may also be used.

The polarization converter 460 may be active and output p-polarizedlight, which may be converted to right-handed circularly polarized (RCP)light after passing the quarter waveplate 470. The active element (AE)421 may receive the right-handed circularly polarized (RCP) light,thereby having the additive state to provide optical power of A.

The SHWP 411 may be active to change the handedness of the circularlylight from left handedness to right handedness, and the active elements(AE) 423 may be in the additive state to provide optical power of R. Theactive elements (AE) 425, 427 and 429 may be in the neutral state (i.e.,zero optical power), and the SHWPs 413, 415 and 417 may be non-activewithout changing the handedness of the light incident thereon. Thus,from t1 to t2, the total optical power of the varifocal structure 400may be continuously adjusted from A+R to A+2R as the voltage applied tothe second LC lens 440 gradually increases.

At the time point t2, the second LC lens 440 may be switched off. TheSHWP 413 may become active, thereby changing the handedness of thecircularly light from left handedness to right handedness, and theactive elements (AE) 425 may enter the additive state to provide opticalpower of R. The active elements (AE) 421 and 423 may be in the additivestate to provide optical power of A and R, respectively. Thus, at thetime point t2, the varifocal structure 400 may have stable optical powerof A+2R.

During T3 stage (t2<t<t3), the polarization converter 420 may benon-active and output p-polarized light, the first LC lens 430 may beswitched-on to provide continuous variable optical power from 0 to R asthe applied voltage gradually increases, the second LC lens 440 may beswitched off. The polarization converter 460 may be non-active andoutput p-polarized light, which may be converted to right-handedcircularly polarized (RCP) light after passing the quarter waveplate470. The active element (AE) 421 may receive the right-handed circularlypolarized (RCP) light, thereby having the additive state to provideoptical power of A.

The SHWPs 411 and 413 may be active to change the handedness of thecircularly light from left handedness to right handedness, and theactive elements (AE) 423 and 425 each may be in the additive state toprovide optical power of R. The active elements (AE) 427 and 429 may bein the neutral state (i.e., zero optical power), and the SHWPs 415 and417 may be non-active without changing the handedness of the lightincident thereon. Thus, from t2 to t3, the total optical power of thevarifocal structure 400 may be continuously adjusted from A+2R to A+3Ras the voltage applied to the first LC lens 430 gradually increases.

At the time point t3, the first LC lens 430 may be switched off, theSHWP 415 may be active, thereby changing the handedness of thecircularly light from left handedness to right handedness, and theactive elements (AE) 427 may enter the additive state to provide anoptical power of R. The active elements (AE) 421, 423, 425 may be in theadditive state to provide optical power of A, R and R, respectively.Thus, at the time point t3, the varifocal structure 400 may have stableoptical power of A+3R.

During T4 stage (t3<t<t4), the polarization converter 420 may be activeand output s-polarized light, the second LC lens 440 may be switched-onto provide continuous variable optical power from 0 to R as the appliedvoltage gradually increases, and the first LC lens 430 may be switchedoff. The polarization converter 460 may be active and output p-polarizedlight, which may be converted to right-handed circularly polarized (RCP)light after passing the quarter waveplate 470. The active element (AE)421 may receive the right-handed circularly polarized (RCP) light,thereby having the additive state to provide optical power of A.

The SHWPs 411, 413 and 415 may be active to change the handedness of thecircularly light from left handedness to right handedness, and theactive elements (AE) 423, 425, 427 each may be in the additive state toprovide optical power of R. The active element (AE) 429 may be in theneutral state (i.e., zero optical power), and the SHWP 417 may be in thenon-active without changing the handedness of the light incidentthereon. Thus, from t3 to t4, the total optical power of the varifocalstructure 400 may be continuously adjusted from A+3R to A+4R as thevoltage applied to the second LC lens 440 gradually increases.

At the time point t4, the second LC lens 440 may be switched off, theSHWP 417 may be active, thereby changing the handedness of thecircularly light from left handedness to right handedness, and theactive elements (AE) 429 may enter the additive state to provide anoptical power of R. The active elements (AE) 421, 423, 425 and 427 maybe in the additive state to provide optical power of A, R, R and R,respectively. Thus, at the time point t4, the varifocal structure 400may have stable optical power of A+4R.

Thus, from t0 to t4, the first LC lens 430, the second LC lens 440, andthe stacked PBP LC lens structure 480 together may provide a continuousadjustment of optical power from A to A+4R for the varifocal structure400.

FIG. 6A is another example of a driving scheme of the varifocalstructure in FIG. 4A, in accordance with an embodiment; FIG. 6B is anexample of an optical path of the varifocal structure having the drivingscheme in FIG. 6A, in accordance with an embodiment; and FIG. 6C is atable showing example optical adjustments in the negative range of thevarifocal structure having the driving scheme in FIG. 6A, in accordancewith an embodiment. The horizontal axis and the vertical axis in FIG. 6Arepresent time and applied voltage to an element in the varifocalstructure, respectively.

Referring to FIGS. 6A-6C, during T1 stage (t0<t<t1), the polarizationconverter 420 may be non-active and output p-polarized light, the firstLC lens 430 may be switched-on to provide continuous variable opticalpower from R to 0 as the applied voltage gradually decreases, the secondLC lens 440 may be switched off. The polarization converter 460 includedin the linear-to-circularly polarization converter 450 may be non-activeand output p-polarized light, and the p-polarized light may be convertedto right-handed circularly polarized (RCP) light after passing thequarter waveplate 470 included in the linear-to-circularly polarizationconverter 450. The active element (AE) 421 may receive the right-handedcircularly polarized (RCP) light, thereby having the additive state toprovide optical power of A.

The SHWPs 411, 413, 415 and 417 may be non-active without changing thehandedness of the light incident thereon. The active element (AE) 423may be in the subtractive state to provide an optical power of −R, andthe other active elements (AE) 425, 427 and 429 may be in the neutralstate (i.e., zero optical power). Thus, from 0 to t1, the total opticalpower of the varifocal structure 400 may be continuously adjusted from Ato A−R as the voltage applied to the first LC lens 430 graduallydecreases.

At the time point t1, the first LC lens 430 may be switched off, and theSHWP 411, 413, 415 and 417 may be non-active. The active element (AE)421 may be in the additive state to provide optical power of A, and theactive element (AE) 423 may be in the subtractive state to provideoptical power of −R. Thus, at the time point t1, the varifocal structure400 may have stable optical power of A−R.

During T2 stage (t1<t<t2), the polarization converter 420 may be activeand output s-polarized light, the second LC lens 440 may be switched-onto provide continuous variable optical power from R to 0 as the appliedvoltage gradually decreases, and the first LC lens 430 may be switchedoff. The polarization converter 460 may be active and output p-polarizedlight, which may be converted to right-handed circularly polarized (RCP)light after passing the quarter waveplate 470. The active element (AE)421 may receive the right-handed circularly polarized (RCP) light,thereby having the additive state to provide optical power of A.

The SHWP 411 may be non-active without changing the handedness of thelight incident thereon. The active element (AE) 423 may be in thesubtractive state to provide optical power of −R. The SHWP 413 may beactive to change the handedness of the circularly light from righthandedness to left handedness, and the active elements (AE) 425 may bein the subtractive state to provide optical power of −R. The activeelements (AE) 427 and 429 may be in the neutral state (i.e., zerooptical power), and the SHWP 415 and 417 may be non-active withoutchanging the handedness of the light incident thereon. Thus, from t1 tot2, the total optical power of the varifocal structure may becontinuously adjusted from A−R to A−2R as the voltage applied to thesecond LC lens 440 gradually decreases.

At the time point t2, the second LC lens 440 may be switched off, theSHWP 413 may be active, thereby changing the handedness of thecircularly light from right handedness to left handedness. The activeelement (AE) 421 may be in the additive state to provide optical powerof A, and the active elements (AE) 423 and 425 each may be in thesubtractive state to provide optical power of −R. Thus, at the timepoint t2, the varifocal structure 400 may have stable optical power ofA−2R.

During T3 stage (t2<t<t3), the polarization converter 420 may benon-active and output p-polarized light, the first LC lens 430 may beswitched-on to provide continuous variable optical power from R to 0 asthe applied voltage gradually decreases, the second LC lens 440 may beswitched off. The polarization converter 460 may be non-active andoutput p-polarized light, which may be converted to right-handedcircularly polarized (RCP) light after passing the quarter waveplate470. The active element (AE) 421 may receive the right-handed circularlypolarized (RCP) light, thereby having the additive state to provideoptical power of A.

The SHWP 411 may be non-active without changing the handedness of thelight incident thereon. The active element (AE) 423 may be in thesubtractive state to provide optical power of −R. The SHWPs 413 and 415may be active to change the handedness of the circularly light fromright handedness to left handedness, and the active elements (AE) 425and 427 each may be in the subtractive state to provide an optical powerof −R. The active element (AE) 429 may be in the neutral state (i.e.,zero optical power), and the SHWP 417 may be non-active without changingthe handedness of the light incident thereon. Thus, from t2 to t3, thetotal optical power of the varifocal structure 400 may be continuouslyadjusted from A−2R to A−3R as the voltage applied to the first LC lens430 gradually decreases.

At the time point t3, the first LC lens 430 may be switched off, theSHWPs 413 and 415 may be active, thereby changing the handedness of thecircularly light from right handedness to left handedness. The activeelement (AE) 421 may be in the additive state to provide optical powerof A, and the active elements (AE) 423, 425 and 427 each may be in thesubtractive state to provide optical power of −R. Thus, at the timepoint t3, the varifocal structure 400 may have stable optical power ofA−3R.

During T4 stage (t3<t<t4), the polarization converter 420 may be activeand output s-polarized light, the second LC lens 440 may be switched-onto provide continuous variable optical power from R to 0 as the appliedvoltage gradually decreases, and the first LC lens 430 may be switchedoff. The polarization converter 460 may be active and output p-polarizedlight, which may be converted to right-handed circularly polarized (RCP)light after passing the quarter waveplate 470. The active element (AE)421 may receive the right-handed circularly polarized (RCP) light,thereby having the additive state to provide optical power of A.

The SHWP 411 may be non-active without changing the handedness of thelight incident thereon. The active element (AE) 423 may be active toprovide an optical power of −R. The SHWPs 413, 415 and 417 may be activeto change the handedness of the circularly light from right handednessto left handedness, and the active elements (AE) 425, 427 and 429 eachmay be in the subtractive state to provide an optical power of −R. Thus,from t3 to t4, the total optical power of the varifocal structure may becontinuously adjusted from A−3R to A−4R as the voltage applied to thefirst LC lens 430 gradually decreases.

At the time point t4, the first LC lens 430 may be switched off, theSHWPs 413, 415 and 417 may be active, thereby changing the handedness ofthe circularly light from right handedness to left handedness. Theactive element (AE) 421 may be in the additive state to provide opticalpower of A, and the active elements (AE) 423, 425, 427 and 429 each maybe in the subtractive state to provide optical power of −R. Thus, at thetime point t4, the varifocal structure 400 may have stable optical powerof A−4R.

Thus, from t0 to t4, the first LC lens 430, the second LC lens 440, andthe stacked PBP LC lens structure 480 together may provide a continuousadjustment of optical power from A to A−4R for the varifocal structure400.

FIG. 7 is varifocal system 700 in which a HMD 705 operates. Thevarifocal system 700 may be for use as a virtual reality (VR) system, anaugmented reality (AR) system, a mixed reality (MR) system, or somecombination thereof. As shown in FIG. 7, the varifocal system 700 mayinclude a HMD 705, an imaging device 710, and an input interface 715,which are each coupled to a console 720. Although FIG. 7 shows a singleHMD 705, a single imaging device 710, and a single input interface 715,in other embodiments, any number of these components may be included inthe system. For example, there may be multiple HMDs 705 each having anassociated input interface 715 and being monitored by one or moreimaging devices 460, with each HMD 705, input interface 715, and imagingdevices 460 communicating with the console 720. In alternativeconfigurations, different and/or additional components may also beincluded in the varifocal system 700. The HMD 705 may act as a VR, AR,and/or a MR HMD. A MR and/or an AR HMD augments views of a physical,real-world environment with computer generated elements (e.g., images,video, sound, etc.).

The HMD 705 may present content to a user. In certain embodiments, theHMD 705 may be an embodiment of the HMD 200 described above withreference to FIGS. 2A and 2B. Example content includes images, video,audio, or some combination thereof. Audio content may be presented via aseparate device (e.g., speakers and/or headphones) external to the HMD705 that receives audio information from the HMD 705, the console 720,or both. The HMD 705 may include an electronic display 255 (describedabove with reference to FIG. 2B), a varifocal block 260 (described abovewith reference to FIG. 2B), an eye tracking module 725, a vergenceprocessing module 730, one or more locators 225, an internal measurementunit (IMU) 215, head tracking sensors 735, and a scene rendering module740.

The varifocal block 260 may adjust its focal length by adjusting a focallength of one or more varifocal structures. As noted above withreference to FIGS. 2B-6C, the varifocal block 260 adjusts its focallength by activating and/or deactivating a SHWP, controlling a state ofa PBP LC lens, adjusting a first LC lens or a second LC lens, somecombination thereof. The varifocal block 260 may adjust its focal lengthresponsive to instructions from the console 720. Note that a varifocaltuning speed of a varifocal structure is limited by a tuning speed ofthe first and second LC lenses.

The eye tracking module 725 may track an eye position and eye movementof a user of the HMD 705. A camera or other optical sensor (that is partthe eye tracking module 725) inside the HMD 705 may capture imageinformation of a user's eyes, and eye tracking module 725 may use thecaptured information to determine interpupillary distance, interoculardistance, a three dimensional (3D) position of each eye relative to theHMD 705 (e.g., for distortion adjustment purposes), including amagnitude of torsion and rotation (i.e., roll, pitch, and yaw) and gazedirections for each eye. In one example, infrared light may be emittedwithin the HMD 705 and reflected from each eye. The reflected light maybe received or detected by the camera and analyzed to extract eyerotation from changes in the infrared light reflected by each eye. Manymethods for tracking the eyes of a user may be used by eye trackingmodule 725. Accordingly, the eye tracking module 725 may track up to sixdegrees of freedom of each eye (i.e., 3D position, roll, pitch, andyaw), and at least a subset of the tracked quantities may be combinedfrom two eyes of a user to estimate a gaze point (i.e., a 3D location orposition in the virtual scene where the user is looking). For example,the eye tracking module 725 may integrate information from pastmeasurements, measurements identifying a position of a user's head, and3D information describing a scene presented by the electronic display255. Thus, information for the position and orientation of the user'seyes is used to determine the gaze point in a virtual scene presented bythe HMD 705 where the user is looking.

The vergence processing module 730 may determine a vergence depth of auser's gaze based on the gaze point or an estimated intersection of thegaze lines determined by the eye tracking module 725. Vergence is thesimultaneous movement or rotation of both eyes in opposite directions tomaintain single binocular vision, which is naturally and automaticallyperformed by the human eye. Thus, a location where a user's eyes areverged is where the user is currently looking and is also typically thelocation where the user's eyes are currently focused. For example, thevergence processing module 730 may triangulate the gaze lines toestimate a distance or depth from the user associated with intersectionof the gaze lines. Then the depth associated with intersection of thegaze lines may be used as an approximation for the accommodationdistance, which identifies a distance from the user where the user'seyes are directed. Thus, the vergence distance may allow thedetermination of a location where the user's eyes should be focused.

The locators 225 may be objects located in specific positions on the HMD705 relative to one another and relative to a specific reference pointon the HMD 705. A locator 225 may be a light emitting diode (LED), acorner cube reflector, a reflective marker, a type of light source thatcontrasts with an environment in which the HMD 705 operates, or somecombination thereof. Active locators 225 (i.e., an LED or other type oflight emitting device) may emit light in the visible band (−380 nm to750 nm), in the infrared (IR) band (−750 nm to 1 mm), in the ultravioletband (−10 nm to 380 nm), some other portion of the electromagneticspectrum, or some combination thereof.

The locators 225 may be located beneath an outer surface of the HMD 705,which is transparent to the wavelengths of light emitted or reflected bythe locators 225 or is thin enough not to substantially attenuate thewavelengths of light emitted or reflected by the locators 225. Further,the outer surface or other portions of the HMD 705 may be opaque in thevisible band of wavelengths of light. Thus, the locators 225 may emitlight in the IR band while under an outer surface of the HMD 705 that istransparent in the IR band but opaque in the visible band.

The IMU 215 may be an electronic device that generates fast calibrationdata based on measurement signals received from one or more of the headtracking sensors 735, which generate one or more measurement signals inresponse to motion of HMD 705. Examples of the head tracking sensors 735include accelerometers, gyroscopes, magnetometers, other sensorssuitable for detecting motion, correcting error associated with the IMU215, or some combination thereof. The head tracking sensors 735 may belocated external to the IMU 215, internal to the IMU 215, or somecombination thereof.

Based on the measurement signals from the head tracking sensors 735, theIMU 215 may generate fast calibration data indicating an estimatedposition of the HMD 705 relative to an initial position of the HMD 705.For example, the head tracking sensors 735 may include multipleaccelerometers to measure translational motion (forward/back, up/down,left/right) and multiple gyroscopes to measure rotational motion (e.g.,pitch, yaw, and roll). The IMU 215 may, for example, rapidly sample themeasurement signals and calculate the estimated position of the HMD 705from the sampled data. For example, the IMU 215 may integratemeasurement signals received from the accelerometers over time toestimate a velocity vector, and integrate the velocity vector over timeto determine an estimated position of a reference point on the HMD 705.The reference point may be a point that may be used to describe theposition of the HMD 705. While the reference point may generally bedefined as a point in space, in various embodiments, a reference pointis defined as a point within the HMD 705 (e.g., a center of the IMU630). Alternatively, the IMU 215 may provide the sampled measurementsignals to the console 720, which determines the fast calibration data.

The IMU 215 may additionally receive one or more calibration parametersfrom the console 720. As further discussed below, the one or morecalibration parameters may be used to maintain tracking of the HMD 705.Based on a received calibration parameter, the IMU 215 may adjust one ormore of the IMU parameters (e.g., sample rate). In certain embodiments,certain calibration parameters may cause the IMU 215 to update aninitial position of the reference point to correspond to a nextcalibrated position of the reference point. Updating the initialposition of the reference point as the next calibrated position of thereference point may help to reduce accumulated error associated withdetermining the estimated position. The accumulated error, also referredto as drift error, may cause the estimated position of the referencepoint to “drift” away from the actual position of the reference pointover time.

The scene rendering module 740 may receive content for the virtual scenefrom a VR engine 745, and provide the content for display on theelectronic display 255. Additionally, the scene rendering module 740 mayadjust the content based on information from the vergence processingmodule 730, the IMU 215, and the head tracking sensors 735. The scenerendering module 740 may determine a portion of the content to bedisplayed on the electronic display 255, based on one or more of thetracking module 755, the head tracking sensors 735, or the IMU 215, asdescribed further below.

The imaging device 710 may generate slow calibration data in accordancewith calibration parameters received from the console 720. Slowcalibration data may include one or more images showing observedpositions of the locators 225 that are detectable by imaging device 710.The imaging device 710 may include one or more cameras, one or morevideo cameras, other devices capable of capturing images including oneor more locators 225, or some combination thereof. Additionally, theimaging device 710 may include one or more filters (e.g., for increasingsignal to noise ratio). The imaging device 710 may be configured todetect light emitted or reflected from the locators 225 in a field ofview of the imaging device 710. In embodiments where the locators 225include passive elements (e.g., a retroreflector), the imaging device710 may include a light source that illuminates some or all of thelocators 225, which retro-reflect the light towards the light source inthe imaging device 710. Slow calibration data may be communicated fromthe imaging device 710 to the console 720, and the imaging device 710may receive one or more calibration parameters from the console 720 toadjust one or more imaging parameters (e.g., focal length, focus, framerate, ISO, sensor temperature, shutter speed, aperture, etc.).

The input interface 715 may be a device that allows a user to sendaction requests to the console 720. An action request may be a requestto perform a particular action. For example, an action request may be tostart or end an application or to perform a particular action within theapplication. The input interface 715 may include one or more inputdevices. Example input devices include a keyboard, a mouse, a gamecontroller, or any other suitable device for receiving action requestsand communicating the received action requests to the console 720. Anaction request received by the input interface 715 may be communicatedto the console 720, which performs an action corresponding to the actionrequest. In certain embodiments, the input interface 715 may providehaptic feedback to the user in accordance with instructions receivedfrom the console 720. For example, haptic feedback may be provided bythe input interface 715 when an action request is received, or theconsole 720 may communicate instructions to the input interface 715causing the input interface 715 to generate haptic feedback when theconsole 720 performs an action.

The console 720 may provide content to the HMD 705 for presentation tothe user in accordance with information received from the imaging device710, the HMD 705, or the input interface 715. In one embodiment, asshown in FIG. 7, the console 720 may include an application store 750, atracking module 755, and the VR engine 745. Certain embodiments of theconsole 720 have different or additional modules than those described inconjunction with FIG. 7. Similarly, the functions further describedbelow may be distributed among components of the console 720 in adifferent manner than is described here.

The application store 750 may store one or more applications forexecution by the console 720. An application may be a group ofinstructions, that when executed by a processor, generates content forpresentation to the user. Content generated by an application may be inresponse to inputs received from the user via movement of the HMD 705 orthe input interface 715. Examples of applications include gamingapplications, conferencing applications, video playback application, orother suitable applications.

The tracking module 755 may calibrate the varifocal system 700 using oneor more calibration parameters and may adjust one or more calibrationparameters to reduce error in determining position of the HMD 705. Forexample, the tracking module 755 may adjust the focus of the imagingdevice 710 to obtain a more accurate position for observed locators 225on the HMD 705. Moreover, calibration performed by the tracking module755 may also account for information received from the IMU 215.Additionally, when tracking of the HMD 705 is lost (e.g., imaging device710 loses line of sight of at least a threshold number of locators 225),the tracking module 755 may re-calibrate some or all of the varifocalsystem 700 components.

Additionally, the tracking module 755 may track the movement of the HMD705 using slow calibration information from the imaging device 710, anddetermine positions of a reference point on the HMD 705 using observedlocators from the slow calibration information and a model of the HMD705. The tracking module 755 may also determine positions of thereference point on the HMD 705 using position information from the fastcalibration information from the IMU 215 on the HMD 705. Additionally,the tracking module 755 may use portions of the fast calibrationinformation, the slow calibration information, or some combinationthereof, to predict a future location of the HMD 705, which is providedto the VR engine 745.

The VR engine 745 may execute applications within the varifocal system700 and receives position information, acceleration information,velocity information, predicted future positions, or some combinationthereof for the HMD 705 from the tracking module 755. Based on thereceived information, the VR engine 745 may determine content to provideto the HMD 705 for presentation to the user, such as a virtual scene,one or more virtual objects to overlay onto a real world scene, etc.

In certain embodiments, the VR engine 745 may maintain focal capabilityinformation of the varifocal block 260. Focal capability information isinformation that describes what focal distances are available to thevarifocal block 260. Focal capability information may include, e.g., arange of focus that the varifocal block 260 is able to accommodate(e.g., 0 to 4 diopters); combinations of settings for SHWPs (e.g.,active or non-active), active PBP LC lenses, and first and second liquidtunable lenses LC lenses that map to particular focal planes;combinations of settings for PBP LC lenses and first and second LClenses that map to particular focal planes; settings for first andsecond LC lenses that map to particular focal planes; or somecombination thereof.

The VR engine 745 may generate instructions for the varifocal block 260,the instructions causing the varifocal block 260 to adjust its focaldistance to a particular location. The VR engine 745 may generate theinstructions based on focal capability information and, e.g.,information from the vergence processing module 730, the IMU 215, andthe head tracking sensors 735. The VR engine 745 may use the informationfrom the vergence processing module 730, the IMU 215, and the headtracking sensors 735, or some combination thereof, to select a focalplane to present content to the user. The VR engine 745 may then use thefocal capability information to determine settings for one or SHWPs, oneor more active PBP LC lenses, the first and second LC lenses, or somecombination thereof, within the varifocal block 260 that are associatedwith the selected focal plane. The VR engine 745 may generateinstructions based on the determined settings, and provide theinstructions to the varifocal block 260.

Additionally, the VR engine 745 may perform an action within anapplication executing on the console 720 in response to an actionrequest received from the input interface 715, and provide feedback tothe user that the action was performed. The provided feedback may bevisual or audible feedback via the HMD 705 or haptic feedback via VRinput interface 715.

FIG. 8 is a process 800 for mitigating vergence-accommodation conflictby adjusting the focal length of an HMD 705, in accordance with anembodiment. The process 800 may be performed by the varifocal system 700in certain embodiments. Alternatively, other components may perform someor all of the steps of the process 800. For example, in certainembodiments, a HMD 705 and/or a console (e.g., console 72θ) may performsome of the steps of the process 800. Additionally, the process 800 mayinclude different or additional steps than those described inconjunction with FIG. 8 in certain embodiments or perform steps indifferent orders than the order described in conjunction with FIG. 8.Additionally, the process 800 may include different or additional stepsthan those described in conjunction with FIG. 8 in certain embodimentsor perform steps in different orders than the order described inconjunction with FIG. 8.

As discussed above in FIG. 7, the varifocal system 700 may dynamicallyvary its focus to bring images presented to a user wearing the HMD 200into focus, which keeps the user's eyes in a zone of comfort as vergenceand accommodation change. Additionally, eye tracking in combination withthe variable focus of the varifocal system 700 may allow blurring to beintroduced as depth cues in images presented by the HMD 200.

As shown in FIGS. 7-8, the varifocal system 700 may determine aposition, an orientation, and/or a movement of HMD 705 (Step 810). Theposition may be determined by a combination of the locators 225, the IMU215, the head tracking sensors 735, the imagining device 710, and thetracking module 755, as described above in conjunction with FIG. 7.

The varifocal system 700 may determines a portion of a virtual scenebased on the determined position and orientation of the HMD 705 (Step820). The varifocal system 700 may map a virtual scene presented by theHMD 705 to various positions and orientations of the HMD 705. Thus, aportion of the virtual scene currently viewed by the user may bedetermined based on the position, orientation, and movement of the HMD705.

The varifocal system 700 may display the determined portion of thevirtual scene being on an electronic display (e.g., the electronicdisplay 255) of the HMD 705 (Step 830). In certain embodiments, theportion may be displayed with a distortion correction to correct foroptical error that may be caused by the image light passing through thevarifocal block 260. Further, the varifocal block 260 mayactivate/deactivate one or more SHWPS, PBP LC lenses, first or second LClenses, or some combination thereof, to provide focus and accommodationto the location in the portion of the virtual scene where the user'seyes are verged.

The varifocal system 700 may determine an eye position for each eye ofthe user using an eye tracking system (Step 840). The varifocal system700 may determine a location or an object within the determined portionat which the user is looking to adjust focus for that location or objectaccordingly. To determine the location or object within the determinedportion of the virtual scene at which the user is looking, the HMD 705may track the position and location of the user's eyes using imageinformation from an eye tracking system (e.g., eye tracking module 725).For example, the HMD 705 may track at least a subset of a 3D position,roll, pitch, and yaw of each eye, and use these quantities to estimate a3D gaze point of each eye.

The varifocal system 700 may determine a vergence depth based on anestimated intersection of gaze lines (Step 850). For example, FIG. 9shows a cross section of an embodiment of the HMD 705 that includescamera 902 for tracking a position of each eye 265, the electronicdisplay 255, and the varifocal block 260 that includes two varifocalstructures, as described with respect to FIG. 2B. As swoon in FIG. 9,the camera 902 may capture images of the user's eyes looking at an imageobject 908, and the eye tracking module 725 may determine an output foreach eye 265 and gaze lines 906 corresponding to the gaze point orlocation where the user is looking based on the captured images.Accordingly, vergence depth (dv) of the image object 908 (also theuser's gaze point) may be determined 850 based on an estimatedintersection of the gaze lines 906. As shown in FIG. 9, the gaze lines906 may converge or intersect at the distance dv, where the image object908 is located. In certain embodiments, information from past eyepositions, information describing a position of the user's head, andinformation describing a scene presented to the user may also be used toestimate the 3D gaze point of an eye in various embodiments.

Returning to FIG. 8, the varifocal system 700 may adjust an opticalpower of the HMD 705 based on the determined vergence depth (Step 860).The varifocal system 700 may select a focal plane that matches thevergence depth by controlling one or more SHWPs, one or more PBP LClenses, first and/or second LC lenses, or some combination thereof. Asdescribed above, the optical power of the varifocal block 260 may beadjusted to change a focal distance of the HMD 705 to provideaccommodation for the determined vergence depth corresponding to whereor what in the displayed portion of the virtual scene the user iscurrently looking.

The foregoing description of the embodiments of the disclosure have beenpresented for the purpose of illustration. It is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description describe the embodiments of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, hardware, or anycombinations thereof.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described.

Embodiments of the disclosure may also relate to an apparatus forperforming the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Embodiments of the disclosure may also relate to a product that isproduced by a computing process described herein. Such a product maycomprise information resulting from a computing process, where theinformation is stored on a non-transitory, tangible computer readablestorage medium and may include any embodiment of a computer programproduct or other data combination described herein.

Finally, the language used in the specification has been principallyselected for readability and instructional purposes, and it may not havebeen selected to delineate or circumscribe the inventive subject matter.It is therefore intended that the scope of the disclosure be limited notby this detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

What is claimed is:
 1. A device, comprising: a first-type liquid crystal(LC) lens configured to provide a first optical power that is variablein a first step resolution; and a second-type LC lens coupled with thefirst-type LC lens, and configured to provide a second optical powerthat is variable in a second step resolution, wherein the first stepresolution is smaller than the second step resolution, and wherein atotal optical power of the device is a sum of the first optical powerand the second optical power, and is variable in the first stepresolution.
 2. The device of claim 1, wherein an adjustment range of thefirst optical power is equal to or larger than the second stepresolution.
 3. The device of claim 1, further comprising: alinear-to-circular polarization converter disposed between thefirst-type LC lens and the second-type LC lens, wherein thelinear-to-circular polarization converter is configured to convert alinearly polarized light output from the first-type LC lens into acircularly polarized light propagating toward the second-type LC lens.4. The device of claim 1, wherein the second-type LC lens includes aPancharatnam Berry Phase (PBP) LC lens.
 5. The device of claim 1,wherein the second-type LC lens includes a plurality of PBP LC lensesand a plurality of switchable half waveplate alternately arranged. 6.The device of claim 1, wherein: the first-type LC lens includes a firstLC lens and a second LC lens arranged in optical series, the first LClens is configured to provide the first optical power for a linearlypolarized light having a first polarization direction, and the second LClens is configured to provide the first optical power for a linearlypolarized light having a second polarization direction that isorthogonal to the first polarization direction.
 7. The device of claim6, wherein the first LC lens and the second LC lens are alternatelyoperated in a switched-on state to provide the first optical power. 8.The device of claim 6, further comprising: a first polarizationconverter, the first-type LC lens being disposed between the firstpolarization converter and the second first-type LC lens, wherein thefirst polarization converter is switchable between outputting thelinearly polarized light having the first polarization direction andoutputting the linearly polarized light having the second polarizationdirection toward the first-type LC lens.
 9. The device of claim 8,wherein the first polarization converter includes a switchable halfwaveplate.
 10. The device of claim 8, further comprising: a secondpolarization converter disposed between the first-type LC lens and thesecond-type LC lens, wherein the second polarization converter is alinear-to-circular polarization converter configured to convert thelinearly polarized light having the first or second polarizationdirection output from the first-type LC lens into a circularly polarizedlight having a same predetermined handedness propagating toward thesecond-type LC lens.
 11. The device of claim 10, wherein the secondpolarization converter further comprises: a switchable half waveplatedisposed between the first-type LC lens and the second-type LC lens; anda waveplate disposed between the second polarization converter and thesecond-type LC lens.